Conventionally, a braking force control apparatus in which a braking force greater than that in a normal case is generated when an emergency braking operation is required has been known, as disclosed in Japanese Laid-Open Patent Application No. 4-121260. The conventional apparatus has a brake booster generating a pressure into which a brake pedaling force F.sub.p is boosted with a predetermined servo ratio. The pressure from the brake booster is transferred to the master cylinder. The master cylinder then generates a master cylinder pressure PM/C corresponding to the pressure generated by the brake booster, that is, corresponding to the brake pedaling force F.sub.p.
In addition, the conventional apparatus has a hydraulic pressure generating mechanism, including a pump as a hydraulic pressure source, generating an assist hydraulic pressure. The hydraulic pressure generating mechanism generates the assist hydraulic pressure based on a driving signal supplied from a control circuit. When the brake pedal is operated at a speed greater than a predetermined speed, the control circuit determines that a driver carries out the emergency braking operation and outputs the driving signal for requesting a maximum assist hydraulic pressure to the hydraulic pressure generating mechanism. The assist hydraulic pressure generated by the hydraulic pressure generating mechanism is supplied to the master cylinder and a change valve. Due to switching operation of the change valve, a higher one of the assist hydraulic pressure generated by the hydraulic pressure generating mechanism and the master cylinder pressure PM/C is alternatively supplied to wheel cylinders.
According to the above conventional apparatus, when the brake pedal is operated at a speed equal to or less the predetermined speed, the master cylinder pressure PM/C which is controlled to a hydraulic pressure corresponding to the brake pedaling pressure FP is supplied to the wheel cylinders. Hereinafter, a control in such a state is referred to as a normal control. In addition, when the brake pedal is operated at a speed greater than the predetermined speed, a high assist hydraulic pressure from the pump which is the hydraulic pressure source is supplied to the wheel cylinders. Hereinafter, a control in such a state is referred to as a brake assist control. Thus, according to the above conventional apparatus, the braking force is controlled based on the bake pedaling force FP. Further, after the emergency braking operation is detected, the braking force can be rapidly increased.
In the above conventional apparatus, to terminate the brake assist control and resume the normal control, it is necessary to switch the change valve so that the hydraulic pressure source for the wheel cylinders is changed from the hydraulic pressure generating mechanism to the master cylinder. The brake assist control is a control for generating a wheel cylinder pressure PW/C sufficiently greater than the master cylinder pressure PM/C when the emergency braking operation is carried out. Thus, when the terminating request of the brake assist control occurs, there may be a great difference between the wheel cylinder pressure PW/C and the master cylinder pressure PM/C.
In a case where the wheel cylinder pressure PW/C greater than the master cylinder pressure PM/C is generated in the wheel cylinders, when the hydraulic pressure source for the wheel cylinders is switched from the hydraulic pressure generating mechanism to the master cylinder, a brake fluid having a high pressure flows backward from the wheel cylinders to the master cylinder. The brake fluid flowing back to the master cylinder passes through the master cylinder and enters a reservoir tank. This phenomenon is not preferable in that unnecessary load is applied to the master cylinder and the pressure in wheel cylinders is prevented from being rapidly decreased.